A frequency synthesizer is an electronic system that translates an input reference frequency signal to an output signal at a different frequency. Specifications such as output frequency range, step size, phase noise, spurious signal levels, and switching speed are important synthesizer characteristics. Trade-offs exist between many different design parameters. As just one example, switching speed, i.e. how fast the output frequency can be changed, may have to be sacrificed to reduce step size and/or phase noise.
FIG. 1 is a simplified block diagram for a conventional PLL frequency synthesizer. The circuit is based on a voltage controlled oscillator (VCO) whose frequency output is locked in relation to a reference frequency by a feedback loop. In the figure, voltage controlled oscillator 125 generates an output signal 135 at frequency f. A portion of this signal is fed back to phase detector 110 via power splitter 130 and frequency divider 140. The frequency divider has a division ratio of N, meaning that its output frequency is N times less than its input frequency. The other input to the phase detector is a reference frequency signal 105 FREF which may be generated by a high-stability, fixed-frequency oscillator, for example. The phase detector compares the two signals at its inputs and generates an error signal which is then fed through low-pass loop filter 115 and amplifier 120 before reaching the voltage controlled oscillator. (The phase detector, low pass filter, amplifier, VCO, power splitter and frequency divider therefore form a loop circuit.) The filtered and amplified error signal changes the frequency of the VCO until f is locked in relation to FREF by: f=FREF×N. If frequency divider 140 is programmable, as is often the case, the synthesizer can be programmed to generate any one of many frequencies within a range determined by the VCO. The step size between the possible output frequencies is equal to the reference frequency FREF.
Phase noise is a manifestation of instability of the output frequency of a PLL synthesizer and is observed as random frequency fluctuations around the desired output frequency. It is a limiting factor in the sensitivity of radio frequency receivers. The level of phase noise near the desired carrier frequency depends on phase noise in the reference signal and on the PLL synthesizer circuit design.
Synthesizer phase noise within the loop filter bandwidth is given by λ=λPD+20 log N where λPD is the cumulative phase noise of the reference signal, phase detector, feedback divider, loop filter and amplifier referred to the phase detector input, and N is the division ratio of the frequency divider. In practice, the synthesizer phase noise performance is usually limited by large division ratios required to provide high-frequency output with fine resolution. For example, to obtain 1 MHz frequency resolution at 10 GHz output, the feedback divider ratio is 10,000, corresponding to 80 dB phase noise degradation.
At high frequencies an additional fixed divider (pre-scaler) may be needed as programmable dividers are often limited to lower frequency operation. This increases the total division ratio by the pre-scaler division ratio resulting in further phase noise degradation. (The amplitudes of spurious signals at multiples of the reference frequency also tend to be proportional to N.)
FIG. 2 is a simplified block diagram for a conventional PLL frequency synthesizer with frequency conversion in the synthesizer feedback loop. The circuit shown in FIG. 2 represents a conventional approach to phase noise reduction in PLL synthesizers based on reducing the frequency division ratio in the feedback loop. In the figure, voltage controlled oscillator (VCO) 225 generates an output signal 235 at frequency f. A portion of this signal is fed back to phase detector 210 via power splitter 230, mixer 250, and frequency divider 240. The other input to the phase detector is a reference frequency signal 205 FREF which may be generated by a high-stability, fixed-frequency oscillator, for example. A digital-to-analog converter (DAC) 260 is provided to translate digital tuning commands 255 for coarse tuning.
Mixer 250 reduces the maximum frequency division ratio by mixing the VCO output frequency with offset frequency f1. Therefore, when the PLL synthesizer of FIG. 2 is locked, f=f1±fREF×N. Offset frequency f1 may be obtained from another phase-locked loop or frequency multiplier.
Circuits of the type shown in FIG. 2 sometimes suffer from false lock to spurious mixer products. For example, the PLL might lock to the wrong sideband, harmonics, intermodulation products or leakage of the local oscillator. An accurate coarse-tuning mechanism is required to avoid false lock problems. In FIG. 2, DAC 260 tunes VCO 225 to approximately the correct frequency before the phase-locked loop locks. For such a coarse tuning system to work well, the tuning characteristics of the VCO must be linear and repeatable. Precise calibration is required to compensate for VCO temperature drift. DAC's are usually noisy and adversely affect synthesizer phase noise performance if they are not properly removed from the circuit after initial frequency acquisition.
Further, in the design of FIG. 2, mixer harmonic and intermodulation products can fall within the synthesizer loop bandwidth as shown in FIG. 3. FIG. 3 shows a spurious mixer product within the low pass filter response of the loop filter. Fractional-N and direct digital synthesis architectures can have similar problems with elevated spur levels.
Another method for generating a high-frequency signal with low phase noise and low spurious characteristics is described in U.S. Pat. No. 7,701,299, incorporated herein by reference. The design of this PLL synthesizer includes a simple, reliable and exact initial tuning mechanism which does not suffer from component instabilities. See FIG. 4. The synthesizer has good suppression of spurious signals because undesired mixer products fall outside the loop bandwidth. Low phase noise performance is achieved by removing frequency dividers from the synthesizer loop.
The initial tuning mechanism in the PLL synthesizer uses a conventional divider loop to lock a VCO to a desired output frequency. Once initial lock is achieved, the divider loop is switched out of the circuit in favor of a low phase noise mixer loop. The design of the mixer loop ensures that spurious signals fall outside the bandwidth of the PLL low pass filter and are therefore easily removed.
FIG. 4 is a simplified block diagram of a low phase noise PLL synthesizer. The circuit is based on a VCO that is locked in relationship to a reference frequency by either of two feedback loops. One loop is used for initial tuning, while another provides low phase noise performance by removing all frequency dividers from the loop.
In the figure, an error signal generated by phase detector 410 is filtered by low-pass filter 415 and amplified by amplifier 420 before feeding voltage controlled oscillator (VCO) 425. A portion of the VCO output signal 435 is split off by power splitter 430 and returned to the phase detector after passing through either of two branches of a feedback loop selected by a switch.
Switch 460 selects either a conventional frequency divider loop similar to that shown in FIG. 1 or a frequency mixer system. A conventional loop comprising divider 440 is selected when switch 460 is in position “1” and is used for initial tuning. This loop includes components in the signal path between signals 463 and 462. A mixer system is selected when switch 460 is in position “2” and is used to achieve low phase noise operation. This loop includes components in the signal path between signals 461 and 464.
In FIG. 4, the mixer system selected by position “2” of switch 460 is illustrated in generalized form. The mixer system comprises: mixers, such as mixers 470, 471, 477 and 478; frequency multipliers, such as multipliers 480, 481, 487 and 488; and frequency dividers, such as dividers 490, 491, 497 and 498. Each mixer has a corresponding multiplier and divider; however, the number of mixers used in a particular system may be one, two, several, or even as many as ten or more. For this reason, FIG. 4 shows mixers denoted M1 through Mi with corresponding multipliers C1 through Ci and dividers D1 through Di. Dotted lines 455 indicate that mixers with corresponding multipliers and dividers may be included in, or removed from, the circuit while maintaining the same architecture and principle of operation. The multipliers' multiplication factors (C1 through Ci) and the dividers' division ratios (D1 through Di) are integers. The multipliers may be comb generators which output a large number of harmonics.
Reference frequency FREF 405 is a high-stability, low phase noise reference signal. FREF is divided by dividers D1 through Di to form phase detector comparison signal 468 (F0) which is one input to phase detector 410. The phase detector compares F0 with signal 466. Note that dividers 490, 491, 497 and 498, providing division ratios (D1 through Di), are not in the PLL feedback loop and are not in the signal path between signals 461 and 464. When switch 460 is in position “2” the VCO clews to a lock frequency given by f=F0(D1D2 . . . Di-1DiCi±D1D2 . . . Di-1Ci-1± . . . ±D1D2C2±D1C1±1). Since all the division and multiplication coefficients are integers, f=F0×N, where N=(D1D2 . . . Di-1DiCi±D1D2 . . . Di-1Ci-1± . . . ±D1D2C2±D1C1±1) is an integer.
Possible frequencies output by the synthesizer of FIG. 4 are equally spaced by F0 as shown in FIG. 5. Switch 460 is set to position “1” to initially tune the synthesizer to one of the frequencies using a conventional divider feedback loop. A desired output frequency can be chosen exactly since divider 440 causes the feedback loop to lock to f=F0×N where N is the division ratio of the divider and N may be chosen to exactly match an output of the mixer branch where N=(D1D2 . . . Di-1DiCi±D1D2 . . . Di-1Ci-1± . . . ±D1D2C2±D1C1±1). This design minimizes the chance of false lock; i.e. locking the loop to an incorrect frequency.
In the design of FIG. 4, the mixer branch does not generate undesired signal products within the synthesizer loop bandwidth. The output of each mixer includes a large number of products including the mixer RF and LO fundamental frequencies, their harmonics, the sums and differences of the RF and LO frequencies, and their harmonics given by fMIX=±mfRF±nfLO which may be written as fMIXi=±mF0N±nF0D1D2 . . . Di-1DiCi for mixer Mi. Assuming that all the coefficients are integers, the mixer products can be expressed as fMIXi=kF0 where k is an integer. Similarly, all harmonic and intermodulation products generated by the mixer branch are multiples of the phase detector comparison frequency F0. These products are easily rejected by a loop low-pass filter. The loop filter bandwidth is made small enough to reject undesired signals, typically ten times less than F0. The output of the PLL is therefore a desired frequency, f, within an effective band pass filter having a width narrower than F0. This design ensures that spurious mixer products fall outside the loop filter bandwidth and are therefore easily removed.
Given a desired output frequency f and frequency resolution or step size F0, the operation of the synthesizer proceeds as follows: Switch 460 is set to position “1” so that the initial-tuning divider branch is connected to, and the mixer branch is disconnected from, the phase detector. The divider ratio, N, of divider 440 is programmed to equal the ratio between the desired output frequency f and the step size F0. The phase detector generates an error signal that tunes the VCO output to f. The phase detector also generates a lock-detect signal that switches switch 460 to position “2”, thereby removing the divider branch from, and including the mixer branch in, the loop circuit. VCO output f is converted in the mixer branch to frequency F0, the same frequency that was generated earlier in the divider branch. The phase detector relocks the output signal f; however, this time there is no frequency division in the feedback loop and phase noise is reduced. Loop low-pass filter 415 removes undesired mixer products thereby ensuring low spurious emissions.
The design of FIG. 4 offers: (1) a simple, reliable and exact coarse-tuning mechanism which does not depend on component instabilities, (2) low spurious signals due to the absence of undesired products within the loop bandwidth, and (3) low phase noise due to removing frequency dividers from the synthesizer loop.
However, in the design of FIG. 4 the frequency step size F0 is limited by the PLL loop filter bandwidth and cannot be arbitrarily small. Reducing F0 leads to smaller PLL loop filter bandwidth and slower switching speed. Thus, there is an inherent trade-off between frequency resolution and switching speed in the design of FIG. 4.
The time spent by a synthesizer transitioning between frequencies is wasted since it cannot be used for data processing. What is needed is a frequency synthesizer that is capable of generating high frequency signals with low phase noise and low spurious signals, yet also has fine frequency resolution and fast switching speed.